Dec 20, 2007: Removed boost::variant
support. After thorough investigation, I think now that the move to make
variant a fusion sequence is rather quirky. A variant will always have a
size==1 regardless of the number of types it can contain and there's no way
to know at compile time what it contains. Iterating over its types is simply
wrong. All these imply that the variant is not
a fusion sequence. (Joel de Guzman)